Studies on the Geological Structure in the Ashibetsu District, Sorachi Coalfleld, Hokkaido

Abstract

The writer has studied about the geological structures in the Ashibetsu district, Sorachi Coalfield since 1941. Fault groups in the district are divided into the following five groups.
(i) The 1st fault group: Reverse fault group with a strike of NNW and an eastery dip. This fault group develops mainly in the central parts of Ashibetsu River district, between Cretaceous outcrops in the eastern side of the Sorachi Anticline and the Bibai-Ashibetsu Tectonic Line, and the Akabira, Tanzan and Raijyo Formations are repeated due to this fault group. This fault group is rare in the eastern and western sides of the Ashibetsu River district.
(ii) The 2nd fault group: Wrench fault group with a strike NW-NNW and an eastery dip. This fault group develops mainly in the western parts near the eastern side of the Sorachi Anticline and in the central parts of the reverse fault zone.
This fault group not only cuts and shifts the folding structures of which axis show NS-NNW, but also shifts northward the strata of the eastern side horizontally. Also, faults are often seen which cut and shift reverse faults.
Near the east side of the Sorachi Anticline, this fault group often changes the strike and decreases its throw towards the cretaceous outcrops. This fault group is not so conspicuous in the eastern side of the Ashibetsu River district.
(iii) The 3rd fault group : Wrench fault group with a NW strike and a west dip. This fault group develops mainly in the eastern parts of the Ashibetsu River district, especially in the Kogane district, and is not so conspicuous in the western side.
(iv) The 4th fault group : Normal fault group with the strike of EW and a dip to north or south. This fault group develops mainly in the Panke-Penkeporonai River district and two kinds of faults with the dip to north or south are seen as a pair. This fault group often cuts the axis of the folding. The strike of this fault group is parallel or oblique to the strike of the strata.
(v) The 5th fault group: Normal fault group with a strike of EW and a dip to south. This fault group develops mainly in the Ashibetsu River district, and its strike is often at a near right angle to the strike of the strata. It cuts often the 1st-3rd fault groups, and is characteristic of the low angle.